1. Field of the Invention
This invention relates to a photovoltaic device to be used as a solar battery or a photo-sensor, etc. and a manufacturing method therefor.
2. Description of Related Art
A gas diffusion method has conventionally been employed to obtain a semiconductor junction, for example, a pn junction in a photovoltaic device such as a solar battery or the like formed of a single crystal material or a polycrystalline material. An example of the gas diffusion method is doping, wherein a single crystalline semiconductor of one conductivity type is placed in an environment containing a dopant impurity element of the other conductivity type a diffusion source, and annealed at high temperatures of about 850.degree.-950.degree. C., whereby the impurity element is diffused into the single crystalline semiconductor. This gas diffusion method is described in detail in Solar Cell Handbook (The Institute of Electrical Engineers of Japan, 1985, pp. 46-47).
Because of the high temperature annealing of the gas diffusion method as described above, a pn junction plane is formed approximately 2000 .ANG. deep from the surface of the single crystal semiconductor. The pn junction plane referred to above is the location where the concentration of the n-type impurities becomes equal to that of the p-type impurities in the semiconductor body. Therefore, in a photovoltaic device having a pn junction formed at a deep location in the semiconductor, most of the charge carriers generated in a relatively shallow depth from the light incidence plane by absorption of the incident light, namely, photo-generated carriers, are lost due to defects in the doping layer, and do not reach the junction plane. Because of this, the conversion efficiency of the photovoltaic device accordingly becomes poor.
Moreover, according to the gas diffusion method described above, the concentration distribution of the impurity element in the vicinity of the pn junction plane changes moderately along the depthwise direction, and therefore the intensity of the internal electric field inside the photovoltaic device becomes small. As a consequence, even if the incident light can reach the junction plane the photo-generated carriers cannot be fully collected due to the low intensity of the internal electric field.
It is known to make the electrode or the semiconductor layer of the photovoltaic device at the light incidence side uneven so as to make more effective use of the incident light. According to this arrangement, the incident light is multi-reflected between the incidence plane of the uneven surface and the back electrode, so that the optical path is elongated. However, since the surface shape of the semiconductor material is not changed as a result of doping of the impurity element in the aforementioned gas diffusion method, the incidence plane has been required to be subjected to a special treatment to be made uneven.